Solar-Powered Hot Cup

Transcription

1 Team Yamagochi International Solar-Powered Hot Cup Phase 4: Performance Validation and Path Forward Michael Brupbacher, Tara Feller, Ian Levy, Craig Sobin, and Alex Vanarelli

2 Contents Introduction... 2 Final Concept Parts and Vendors... 3 Assembly... 4 Testing Procedure... 7 Testing Results... 9 Prototype Testing Results vs. Project Wants and Metrics Project Cost Mass Production Cost Estimate Path Forward Conclusion

3 Introduction The CEO and founder of the Yamagochi International Corporation, Gordon Miles, has presented the task of designing a solar Warm-a-Matic heating system. The system should be completely powered by solar energy, and should be able to be implemented in a travel size cup. The solar-powered hot cup should be able to safely apply heat to a contained liquid in order to combat heat loss to ambient conditions. The idea of the solarpowered Warm-a-Matic system was developed by Gordon Miles while observing people in a cold outdoor environment throwing away left over coffee due to a drop in the liquid s temperature. The Warm-A-Matic system should be designed with commercial production in mind, and the general concept should be extendable to future applications. Gordon Miles will act as the sponsor providing the funding and budget for this project. Understanding that Gordon is an entrepreneur, the prototype will be designed with components that are as inexpensive as possible while maintaining functionality. With parts selected for the final concept, the next step in the design process was to order and assemble the parts into a viable prototype of the solar powered hot-cup. After the working prototype was constructed, the prototype was tested based and compared to the wants and metrics generated at the beginning of the design process. This report will include a specific parts list from specific vendors and will outline the assembly of the prototype. In addition, results from testing will be presented and suggestions will be made for the path forward of this technology. 2

4 Final Concept Parts and Vendors Table 1. Parts and Vendors List Component Part Vendor Flexible Film Solar Cells RC PSA 7.2 Volt 100mA 3.5 x 10.6 Flex solar Cells Lithium Ion Battery Customized Li-Ion Battery: 3.7 Volts 940mAh with PCB Battery Space SPDT Switch SPDT on/off/on switch Sigma Switches Blocking Diode Blocking Diode 1A 40V DC for Solar Panels alte store Resistive Heater Wire 24 AWG Nickel 200 Resistive Heater Wire Pelican Wire Aluminum Foil.002 Thick Aluminum Foil AF Quick Ship Metals Adhesive Highly Filled Thermally Conductive Epoxy Rubber. Epoxies Etc. Snap Action Thermostat 36T Thermodisc Snap-Action Thermostat Testco-inc. Stainless Steel 16ga (.060") 304 Stainless Steel Sheet #2 Finish-12"x12" Plate 2-1/2" OD {A} x 2.370" ID {B} x.065" {C} Wall Tube 304 Stainless Steel Speedy Metals 3-1/2" OD {A} x 3.370" ID {B} x.065" {C} Wall Tube 304 Stainless Steel Polypropylene Custom SLA with ACCURA 25 Resin Metro Rapid Prototyping Polyurethane Foam AeroMarine 2# Density Polyurethane Foam JGreer AeroMarine Products When selecting the parts for the working prototype, there were many different aspects that factored into the decision making process. All of the parts must be compatible with one another to create a prototype that will comply with the given project scope as best as possible, and the parts must be as inexpensive as possible in order to keep to the prototyping and manufacturing cost limitations set by the sponsor. The flexible thin film solar cells that were purchased for the application were the best available without resorting to ordering custom solar cells. The panel has an operating voltage of 7.2 volts which had to be above the voltage of the 3.7 volt lithium ion battery in order to be able to charge the battery. The 100mA operating current would allow for a faster charging time than the lower current panels on the market. The panel is as close to covering the outer shell surface area as possible for existing products, while providing the correct voltage and current needed to charge the lithium ion battery. Knowing that the operating voltage and current would drop due to the panel being wrapped around a curved surface, the 7.2 volt panel was chosen rather than a 6 or 4.8 volt solar panel. The lithium ion battery that was chosen for this application was chosen mainly based on its high energy density. The battery had to have a voltage low enough to be charged by the 7.2 volt flexible solar panel and therefore the 3.7 volt lithium ion batteries on the market were chosen. The battery had to have a high capacity in order to provide current to the heating element for a longer period of time, while being low enough to ensure 3

5 a reasonable charge time. The current provided by the battery also had to be high enough to generate large amounts of heat by means of electrical resistance. Lastly, the battery had to fit in the electronics housing located in the bottom of the outer casing of the prototype. A simple switch was chosen to allow the user to be in control the different functions of the hot-cup. Due to the fact that the battery is unable to be charged by the solar panel and discharge into the resistive heater wire simultaneously, the switch must have two different on settings. One setting must connect the solar panel to the battery in order to charge the battery and one setting must connect the battery to the resistive heater in order to provide the electrical energy to supply heat to the contents of the hot-cup. In order to do this, a Single Pole- Double Throw switch can be used. With an SPDT switch, there is also an off setting in which neither of the cups functions are being used. A blocking diode is necessary in this design in order to make sure that the battery does not discharge back into the solar panel when the panel in the shade. The blocking diode has a sufficient power rating for this application. After researching custom foil heater vendors, it was apparent that the custom heaters would be too expensive for this application. Therefore it was decided that the components of a foil heater would be purchased and the heater would be built in-house for the prototype. Therefore, resistive heater wire, thick aluminum foil, and a thermally conductive and electrically insulating adhesive would be needed in order to construct the heating element. Nickel 200 wire with fiberglass electrical insulation was chosen based on its low resistance per foot. A lower resistance per foot would allow for more wraps of the heater wire around the circumference of the stainless steel inner liner providing a more uniform heating surface. A thermally conductive adhesive is necessary in order to hold the resistive heater wire in place while still allowing the heat to be conducted from the heater wire to the inner liner. The.002 inch thick aluminum foil also allows for a more uniform heat flux through the stainless steel inner liner. The snap action thermostat was chosen based on its cutoff temperature of 158 F and based on its reset temperature of 131. Since a temperature range of F was listed in the design metrics of the hot-cup, this snap-action thermostat would not allow the contents of the cup to surpass the upper limit of the temperature range to a temperature that is unsafe. In order to have a stainless steel inner liner with a uniform thickness, two stainless steel cylinders with equal thickness but differing diameters were chosen. The cylinders were ordered with the specific diameters of the upper and lower section of the inner liner design. A stainless steel plate with the same thickness was purchased as well in order to combine the two cylinders into the shape of the inner liner. Metro Rapid Prototyping is a rapid prototyping company that uses stereolithography in order to produce a quality resin prototype. They use ACCURA 25 resin which accurately mimics the thermal and physical properties of polypropylene, which was the chosen outer casing material for the hot-cup design. Assembly First, the solar panels were arranged on the outer casing where they would eventually be adhered. Two pencil marks were be made on the outside of the outer casing where two holes were drilled in order to allow the 4

6 leads of the solar panels to connect to the rest of the circuit. The solar panels were then fixed to the outside of the outer casing with the pressure sensitive adhesive already on the back of the panel and the leads of the solar panels were fed through the drilled holes in the outer casing. Figure 1: Solar Panels on Prototype The stainless steel inner liner was then constructed. A lathe was used to cut the cylinders to the appropriate lengths and a CNC milling machine was used to cut the flat stainless steel plates to the desired annulus. These stainless steel parts were then welded together in the desired arrangement in order to create a liquid tight inner liner. Excess metal from the welding process was removed with the use of a lathe and the inner liner was then cleaned using steel wool and the liner was filled with water to make sure that it was still liquid tight. In order to construct the foil heater, the resistive wire was cut to a length where the resistance of the system would be 2.11 ohms. This was calculated using ohm s law where V=IR, V was the 3.7 output voltage of the battery, and I was 1.75 amps of output current. It was recommended by the manufacturer that the battery was not discharged above 1.8 amps. The wire was wrapped tightly around the outer surface area of the inner liner and was secured after each wrap using a small amount of hot glue. A small length of wire on each end was left unwrapped in order to function as leads for the resistive heater. Once fully wrapped, correct amounts of the rubber epoxy and a catalyst that reacts with the rubber epoxy to start the curing process were calculated and mixed together. A thin layer of the thermally conductive epoxy was spread over the entire surface of the attached wire. A single layer of the thick aluminum foil was wrapped around the adhesive and several rubber bands were used to hold the heater unit in place during the epoxy s 24 hour cure time. The same epoxy was used to adhere the snap action thermostat to the bottom of the inner liner. 5

7 Figure 2: Construction of Foil Heater The electrical circuit of the heating system was then constructed using recycled lead wires. The circuit was arranged through the holes in the outer casing to and from the electronics housing in the bottom of the cup and to the switch. All of the lead wires were soldered to ensure durable and reliable connections. The following diagram shows the wiring circuit that was used: Figure 3: Wiring Diagram Once the circuit was correctly wired and tested, the inner liner was inserted into the outer casing. The holes leading from the inside of the outer casing to the electronics housing were closed using a quick setting RTV silicone gasket paste. Knowing that the foam would expand to 25 times its original volume once the two parts were combined, appropriate amounts of each part of the polyurethane insulation were calculated and mixed and the expanding polyurethane foam insulation was poured in between the outer casing and the inner liner. 6

8 Once the foam expanded upward, close to the top of the outer casing, the upper ring was inserted in order to enclose the area in between the outer casing and the inner liner. Testing Procedure The testing procedure for the validation of the prototype metrics consisted of multiple experiments. First, the battery charging capability of the flexible thin film solar panel was assessed. In order to do so, the prototype was placed in sunlight for an extended period of time. The voltage of the battery and both the current and voltage output of solar panel were monitored in 5 minute increments. With this data, the ability of the solar panel to charge the lithium ion battery can be validated in plot form. An estimate regarding the charge time to regain the full capacity of a dead battery can also be generated from this data. Figure 4: Testing of Solar Panel After charging the battery to full capacity using an external current source, the heating ability of the thick film resistive heater was tested. In order to do so, the hot-cup was filled with 24-ounces of water at the maximum temperature defined by our metrics, 165F. Using the SPDT switch, the circuit was then switched to the heating setting at time t=0. A digital temperature sensor was then then used to record the temperature of the cups contents in 5 minute increments. This experiment showed the discharge time of the batteries as well as the effectiveness of heating element that is applied to the outside of the inner liner. Specifically, this test showed whether the cup met the predetermined metric of maintaining a 135F to 165F liquid temperature range and a battery life of 1-2 hours. 7

9 Figure 5: Testing of Heating System The thermal cut-off by means of a snap action thermostat was also tested. This was done by filling the cup with water above the thermostat s specified cut-off temperature while tracking the current from the battery to the resistive heater using a multi-meter. Figure 6: Testing of Snap-Action Thermostat 8

10 Current (ma) Testing Results The data collected for the output of the solar panel was taken during the first hours of sunlight. Although the intensity of the incident radiation of the sun on the earth s surface is a maximum at solar noon, the test was conducted in the early morning hours when environmental conditions were optimal at this time of year. The following plot shows the current output of the solar panel for a one hour test compared to the output under alternate conditions: 120 Solar Panel Output Time (min) Current output with dead cells (ma) Current output without dead cells (ma) Ideal Current Output Figure 7: Solar Panel Output Although the voltage of the solar panel and the voltage of the battery were monitored and recorded during the test, the fact that current was flowing for the duration of the test implicitly validates the condition that the voltage of the solar panel was above the voltage of the battery. From figure 7 it is apparent that the current output of the solar panel was significantly lower than the operating current of the panel specified by the manufacturer. Although this power loss is partially due to non-ideal solar conditions and the fact that the panel was mounted around a curved surface, it can mostly be attributed to dead cells on the panel. As a result of prototyping cost limitations, a stock panel without an optimal wiring configuration was purchased for the prototype. This resulted in shaded cells on the panel and a large loss in power as a result of forcing current through these dead cells. In order to simulate the output of a panel without this relatively large power loss, light was reflected onto the dead cells. The current output of the panel increased dramatically and the series Current output without dead cells plots the largest value noted while reflecting light onto the dead cells. Using this value for an estimate of a custom solar panel with an optimal wiring configuration, the battery charging time was estimated to be 16 hours. The following graph shows the temperature vs. time profile of the cups contents with and without the activation of the heating element through the entire lifetime of the battery: 9

11 Cureent (A) Temperature (Degrees Fahrenheit) Heat Loss y = x y = x Time (Minutes) No Heater With Heater Battery Depletion Linear (No Heater) Linear (With Heater) Figure 8: Temp vs. Time Comparison In comparing the series No heater with the series With Heater, the effectiveness of the heater can be noted. Although the heater did not reduce heat loss from the cup drastically, there is an increase in the rate of temperature reduction when comparing the test without heater activation to the test with activation. This is seen mathematically by comparing the slope of the linear regression lines for the two series. The battery life was also obtained from this test and was approximately 35 minutes. The following plot shows the current supplied to the heating element during the testing of the snap action thermostat: Current vs. Time T= 172 F 3.12 T = 138 F Time (min) Snap Action Test Figure 9: Snap Action Thermostat Test 10

12 From figure 9 it is apparent that current was supplied to the heating element until the cups contents reached a temperature of 172F, when the snap action thermostat was activated and the circuit between the battery and the resistive heater was disconnected. From figure 9 it is also apparent that the response time of the snap action thermostat was 3 minutes. The circuit between the battery and the resistive heater remained disconnected until the fluid temperature cooled to 138F, at which point the circuit was reconnected and current was supplied from the battery to the resistive heater once more. Although we did not have a metric for the response time of the safety cut-off, 3 minutes was longer than expected. This can be attributed to the fact that the inner liner was made from stock tube and was thicker than necessary. In mass production the inner liner can be constructed with metal stamping and will be significantly thinner. This will increase the response time of the snap action thermostat significantly. Prototype Testing Results vs. Project Wants and Metrics The testing results were mixed with successes and areas that are still in need of improvement. The proposed mass production cost was originally $ The prototype s estimated mass production cost was $ This estimation does not include various starts up prices, such as the cost of a mold for injection molding and other custom parts necessary for the product. The target temperature range to maintain was 135F to 165F. The prototype was able to hold hot liquid within that temperature range for 85mins. The metrics never defined a time limit for this, though keeping coffee hot for nearly an hour and a half is quite satisfactory. Another metric successfully met was that the cup was dimensioned to fit into a standard vehicle cup holder (3 ½ diameter, 2 ½ depth). Fully discharging the battery into the heater takes approximately 40 minutes. The metric regarding battery life was 1-2 hours, but the battery was unable to meet this metric due to a need for a relatively low capacity in order to fully charge using low current output solar panels in a reasonable amount of time. The prototype had a metric of being a 24oz container, which it successfully meets. And finally, it was proposed to have a weight of no more than 3.5 pounds, and the final weight was just over 3 pounds. Project Cost The final prototype cost was well below the budget presented by the sponsor. The initial budget was $2000, while a refined budget was just over $1500 and our total for parts came to $ The largest contributor to the total cost of the prototype was the $981 cost of the rapid prototyping by Metro Rapid Prototyping. Flex Solar Cells provided the flexible film solar panel for $ For testing purposes two lithium ion batteries were purchased for a total of $23 from Battery Space. Due to the fact that a large startup cost and mass production quantity order was needed in order to acquire a foil heater, the components needed to construct a foil heater were ordered. A 50 foot spool of heating wire from Pelican Wire was purchased for $45 and thick aluminum foil from Quick Ship Metals was ordered for $ The thermally conductive rubber epoxy for the foil heater was provided as a free sample. The smallest volume available, a half a gallon kit of pourable polyurethane foam insulation was found at JGreer AeroMarine Products for $20. The blocking diode from alte store was $6.32 and a free SPDT switch was obtained from Sigma Switches. Due to the fact the minimum order was $25.00, seven snap action thermostats were ordered from Testco. The stainless steel for the inner liner was ordered from Speedy metals for $ O-rings to seal the electronic compartment and the lid cost from All-O-rings. The costs of all of the prototype components added up to equal $ and the total shipping cost equaled $77.93, bringing the grand total of the prototype to $

13 There are two implicit costs that are not reflected in the total, the time spent in the machine shop and engineering fees to design the prototype. A total of 5 hours were spent in the machine shop totaling $50 at $10 per hour. For non-professional engineers, $20 per hour per person is an estimate for engineering design/labor. On average 20 hours per week per person were spent over the 15 week period designated to designing the prototype. For all 5 team members, this comes to $30,000 or $6000 per person to design the prototype. The implicit costs total $30,050 bringing the true cost for the prototype to $31, Mass Production Cost Estimate To calculate what the product would cost in mass production, research was done to see what custom parts would cost. The cost per unit also decreases dramatically as the quantity goes up. There are overseas companies that have a minimum order of 1,000 units or more. The two parts of the design that must be custom made for mass production are the solar cells and the polypropylene outer casing. The solar cells need to be custom so that the size is optimized and are wired correctly to eliminate the effect of dead cells on the charging current. A typical start-up cost for custom orders is around $5000. Depending on the company producing the custom order, prices per unit can be estimated at $6.00. The polypropylene outer casing is injection molded for mass production. Creating a mold and start-up cost can be estimated to $5000, but the cost per unit goes down to around $2.00. All of the other parts can be ordered from other companies in mass quantity. Stainless steel stamping cost is estimated to be $4.25 for the amount needed. The lithium ion battery s price doesn t decrease due to a different method of production, but purely on the price dropping due to the order size. A lithium ion battery can be obtained for $3.50 for large orders. Blocking diodes don t change much in price, but do drop to $1.00. Due to the fact that such a small amount of foam is needed for each unit, only $0.10 is needed in polyurethane foam for each product. Several foreign companies produce large orders of foil heaters, snap action thermostats, and switches. Foil heaters can be ordered for a wide range of specifications and range up to $1.50 per unit. Companies were found that sell snap action thermostats as low as $0.30, and three-way-switch for $0.10. For mass production, the average cost is approximately $18.75 plus the cost to start up custom orders of solar panels, and creating a mold for the polypropylene. Path Forward There are several areas of this design that can be improved. First off, the effects of the heater were minor. This is mainly a result of the inability to buy a thick film heater on the proposed budget, as previously stated. The solution here would be simple: to get a professionally mass produced foil heater that is made custom for the cup. These costs would not be so inordinate at mass production level. Another area that needs improvement is in the design of the solar panels. With the current setup, it is a very slow process to charge the battery unless there are sources of light reflection on the cells that are not in the sun. This is a result of the wiring of the solar panel being in series along the circumference of the cup. There will be a certain number of cells in each series that will be in the path of incident light, which will create a large power loss. In order to charge the battery in a reasonable amount of time, a custom thin film solar cell would necessary. When the cells are forcing their energy through dead cells, it significantly diminishes the current and voltage output of the entire cell. Figure 10 shows how the cells currently are, the yellow X s represent the dead cells, and the arrow represents the current. 12

14 i Figure 10: Current Direction in Solar Panel (A) The power loss associated with the dead shaded cells can be regained with a custom flexible thin film solar panel design. The solution here would be to have a custom panel designed that would be wired in parallel, as shown in figure 11. With custom thin film solar panels, the size of the module can be optimized cover the largest amount of surface area present on the upper section of the outer shell. i Figure 11: Current Direction in Solar Panel (B) A method of significantly reducing heat loss to the surroundings is necessary to increase the functionality of this design. Although prototype cost considerations ruled out vacuum insulation, the cup design can be altered to accommodate for a change from polyurethane insulation to vacuum insulation. With vacuum insulation in an ideal situation, the heat loss from a uniform temperature medium would be reduced by 75%, as seen in figure 13. The prototype design could be easily augmented to allow for this method of insulation. Although cost prohibited the use of vacuum insulation for the prototype, the cup design could be easily modified to implement this technology in mass production. 13

15 Q Q Q Q Figure 12: Heat loss without vacuum insulation Q Figure 13: Heat loss with vacuum insulation Conclusion The design, assembly, and testing of the prototype has illustrated the viability of the Warm-A-Matic heating system. With the proposed path forward, the design could be improved and be ready for mass production. 14